using free software to scan external drive for spyware

i normally use free software like adaware to scan for spyware. to scan external hard drive, we need to purchase the license. is there any way to scan external hard drives for spyware using free software?

Comodo Antivirus has all the functionality of a paid AV without the price its free forever! No licenses, upgrades, or any hidden costs.
One of the features is
Scan Removable Devices - CD's, DVD's, external drives, USB devices, digital cameras morehttp://antivirus.comodo.com/